1. Field of the Invention
The present invention generally relates to two-stroke internal combustion engines, and more particularly, to engines having a relatively small displacement and that can be used on a portable power working machine.
2. Related Background Art
In small two-stroke internal engines, all the strokes including suction, compression, expansion and exhaust strokes are controlled by a piston-ported control system. During the exhaust stroke, the combustion chamber is scavenged of burnt gas from an air-fuel mixture including fuel and lubrication oil. More specifically, a fresh air-fuel mixture is first introduced into a crank chamber in which it will be compressed by a descending piston (during the expansion stroke). Subsequently, in the exhaust stroke, the combustion chamber is scavenged of burnt gas from the fresh air-fuel mixture compressed in the crank chamber and introduced into the combustion chamber.
There are known ways of scavenging, one being a method of introducing a fresh air-fuel mixture in the crank chamber directly into the combustion chamber through a scavenging passage formed in the inner wall of a cylinder, and another being a method of introducing a fresh air-fuel mixture into the combustion engine via apertures formed in the circumferential surface of the piston as disclosed in the Japanese Utility Model Laid Open No. 33232 of 1982 (patent document 1) and Japanese Patent Laid Open No. 1359 of 1985 (patent document 2). The piston having such apertures is called an “internal cooling piston”.
Note here that because of worldwide concern, more environmentally friendly devices are in demand, and regulations have not only been imposed upon the emission of exhaust gas from automotive and motorcycle engines but also from two-stroke internal combustion engines having a relatively small displacement and used on a knapsack-type duster or chain saw. On this account, Japanese Patent Laid Open No. 2002-227652 (patent document 3) discloses a two-stroke internal combustion engine capable of reducing the amount of a so-called blow-by phenomenon, namely the phenomenon that a part of an air-fuel mixture, that is to be used for scavenging during the exhaust stroke, is discharged from the combustion chamber to outside the engine without contributing to the combustion. More particularly, this patent document 3 proposes to narrow the inlet of the scavenging passage formed in the inner wall of the cylinder. In other words, patent document 3 proposes to narrow the inlet opening area of the scavenging passage which is adjacent to the crank chamber compared to the cross-sectional area of the other part of the scavenging passage.